Filtering unit and method of sealing same

ABSTRACT

Ceramic filter elements ( 1, 15, 26 ) have a feed surface covered with a ceramic filter membrane ( 4, 19 ). As a feed liquid flows over the feed surface, a portion of the liquid passes through the membrane and the filter and is discharged through a discharge surface as a permeate liquid flow. Between the feed and discharge surfaces, the filter element includes an impermeable surface portion ( 8, 24, 30 ) which contacts a seal or gasket assembly ( 17, 29 ). The seal or gasket assembly includes two seal portions ( 9, 10 ) which define a channel ( 12, 22, 31 ) in between. The channel connects with a discharge channel ( 11, 23, 32 ). In this manner, any liquid leaking between the filter element and another filter element or a housing ( 21, 27 ) enters the discharge channel ( 11, 23, 32 ). Liquid in the discharge channel indicates a leak in the seal and potential contamination of the permeate liquid.

FIELD OF THE INVENTION

The invention relates to filtration by means of crossflow technique andfiltering elements having a filter membrane on a porous carrier.

Particularly, the invention relates to a crossflow membrane filteringunit having a sealing system including a channel enabling the detectionof a leak without the quality of the filtrate being impaired.

BACKGROUND OF THE INVENTION

The removal of solids from a liquid by filtration can take place inseveral, principally different manners. In so-called dead endfiltration, a suspension is driven in its entirety towards a filter, anda filter cake is formed which the filtrate penetrates. In cross-flowfiltration, the mixture to be filtered is pumped across the filterclement in a circuit, whereby part of the mixture penetrates thefiltering elements forming a permeate stream. In the ideal case, no cakeis formed, but the material to be separated is enriched in thecirculating phase, and a certain fraction is continuously being removedtherefrom as a reject stream. The quality of the permeate is basicallydependant on the pore size of the filtering element membrane. This poresize can be very narrowly defined. In microfiltration, the pore diametercan be 0.1-10 μm, and in ultrafiltration down to 1 nm. Thus, separationof molecules on the basis of size is possible by means of filtration.One application is the removal of pyrogens from water, pyrogens havingrelatively large molecules.

Membrane filtration elements have porous carrier structures, on thefiltering surfaces of which membranes having well defined pore sizeshave been formed.

Membranes with small pore sizes can be formed from e.g. polymer films.These are not, however, particularly resistant to mechanical and thermalstress, which is a drawback in sterilisation and cleaning. Ceramicmembranes on porous, ceramic carrier phases have also been developed.Most ceramic materials resist heat in addition to being mechanicallydurable. On a ceramic element, as on other materials, surfaces can alsobe sealed, i.e. made impermeable. Thus, three types of surfaces mayoccur on a ceramic filter element: membrane coated, sealed, anduntreated, through which permeate flows freely.

For example, in U.S. Pat. No. 5,104,546 a multilayer ceramic tubularfilter element is disclosed, suitable for pyrogen removal byultrafiltration. The end walls of the element as well as the inner wallsof the channels are membrane coated. When the element is fitted into ashell or housing, and a mixture is pumped through the inner channels,permeate flows through the porous carrier to the shell side. Seals, forexample rubber gaskets, must be provided between the shell and theelement end walls.

In addition to tubular elements, plate-shaped ceramic elements aregenerally known, see for example WO 98/28060 and references therein. Aplate-shaped filter element has a membrane coated outer surface and anaperture in the centre or elsewhere, the walls of which aperture are notmembrane coated. Filters can be assembled by stacking such plates, asshown in e.g. GB 1 268 875. Due to the shape of the plates, the matingsurfaces may be higher than the filter surfaces, whereby free filterarea is provided between the plates as stacks are formed; plates may beseparated by spacers; or if the seals between plates are thick enough,the seals function as spacers. Thus, the central apertures form apermeate retrieval and exit channel, and the plate stack forms aflanged-tube structure across which the pressurised mixture to befiltered is allowed to flow. The mixture thus flows on the shell sideand the permeate is collected from the inner channel.

As stacks are assembled, seals must be used between the plates and/orthe spacers, and between the elements and the housing, the number ofseals being proportional to the number of plates used.

FIG. 1 shows a filter plate 1 according to the prior art, having acentral bore 2. FIG. 2 shows a section of part of a stack formed fromplates 1. Four plates are shown in the Figure, but the stack maycomprise additional plates depending on required filter surface area;the stack is confined in a housing not shown in the Figure. In thehousing, a flow of pressurized mixture for filtration is maintained. Theplates comprise a porous ceramic support body 3, formed from e.g.,aluminum oxide and having a relatively large pore size, e.g., >1 μm, andon the surface of the support body has been formed a ceramic filtermembrane 4. The membrane has been formed from e.g., titanium orzirconium oxide. The central bores 2 of the plates form a permeatechannel 6; in the embodiment shown, a perforated tube 5 is providedtherein. The joints between the mating faces of the plates are sealed bymeans of O-rings 7.

During the filtration process, bodies smaller than the pores of membrane4, as e.g. water molecules in the case of ultrafiltration, pass throughthe membrane due to the pressure difference, and are carried through theporous support body into the permeate chamber and exit through the endfittings.

In the above described filter units, the seals between the mixture andthe permeate side are potential leak sites, and in demanding filteringprocesses a small leak may immediately render the permeate useless, asthe mixture to be filtered penetrates into the permeate channel.

In tube or tube bundle type ceramic filter units, the sealing problem isassociated with the end sections of the units. If the feed mixturepenetrates the joint between the housing and the filter clement andenters the shell side, the permeate is contaminated. In plate stack-typefilter units, potential leak sites are the joints between the plates andbetween the plate stack and the housing.

SUMMARY OF THE INVENTION

A main object of the invention is a crossflow-type membrane filteringunit wherein the sealing system between the filtering element and itshousing comprises a channel through which tho feed mixture onpenetrating the seal system can flow without penetrating the wholesealing system, thereby indicating a leak.

In an embodiment using plate-shaped filtering elements, two concentricseals are preferably used between the filter plates. Thus, a spacedelimited by the seals and two plates is formed. At the location of saidspace, the plate surface is sealed, that is made impermeable. Inaddition, at the location of said space, at least one opening isprovided in each plate, interconnecting the spaces between adjacentplates in the stack. The walls of the opening(s) are also scaled, andconsequently an isolated, sealed space surrounding the permeate channelis formed. A tube or equivalent can be provided to connect this space tothe outside. If a leak occurs in a seal between two plates, the fluidmay thus flow into the isolated space and out. Thus, a leaking seal canbe detected immediately, and no possibility exists for the feed mixtureto enter the permeate channel.

Preferably, the double seal system consists of two concentric, annularseals of different diameter. The seals may be of the O-ring type, andcorresponding grooves may be provided in the filter plates. A huge rangeof O-ring seals designed for various chemical and physical conditions isavailable, and as the chemical and thermal durability of ceramic filterplates is usually high, a filter structure is thus provided which isfunctional in harsh conditions, with a low risk for permeatecontamination by feed mixture.

In embodiments comprising filter elements of a tube or tube bundlestructure, the joint between the filter housing and the filter elementmust be reliably sealed. The seal can be formed between the element endwall and the housing, or between the side walls of the element and thehousing, and appropriate surface sealing of the filter element at thesite of the seal system must be provided.

Surface scaling can be effected in various manners known to the personskilled in the art. On ceramic filter elements the surface may beglazed, or organic coatings may be used according to the processrequirements. Organic coatings are obviously useful in the case oforganic filtering elements. Metal foils is another possibility.

A further object of the invention is to provide a method of sealing acrossflow membrane filtering unit comprising one or more filteringelements within a housing.

Preferably, filtering units according to the invention are used in waterfiltration, more preferably water ultrafiltration; most preferablyfiltering units according to the invention are used in the production ofheat sterilised, pyrogen-free water.

Still further advantages of the present invention will become apparentto those of ordinary skill in the art upon reading and understanding thefollowing detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1 shows a ceramic filter plate according to the prior art;

FIG. 2 shows a sideways section of a plate stack according to the priorart, assembled from plates;

FIG. 3 is a schematic overview of a crossflow filtration system;

FIG. 4 shows a ceramic filter plate according to the present invention;

FIG. 5 shows a sideways section of a plate stack according to thepresent invention, assembled from plates according to claim 4;

FIG. 6 shows an embodiment of the present invention, wherein a tubulartype filter element is used and the seal is located between the end wallof the element and the head of the housing;

FIG. 7 shows an embodiment of the present invention, wherein a tubulartype filter element is used a nd the seal is located between thesidewalls of the element and the housing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is described below with reference to the encloseddrawings.

In FIG. 3, an overview of crossflow filtering system is shown. Materialto be filtered enters through the feed line 5, by means of pump 1. Pump2 serves the filtration circuit, in which the mixture is pumped acrossfilter unit 3. Filter unit 3 can be of any suitable type, e.g. the platestack type or the tube or tube bundle type mentioned above. Part of thecirculating fluid penetrates the pores of the filtering elements,forming the permeate, which eaves through line 6 or alternativelythrough reject line 7. To keep the concentration of retained material inthe filtration circuit at a desired level, a retentate stream iswithdrawn through line 8, while the balance remains in the filtrationcircuit 9.

FIG. 4 shows a filter plate according to the invention in a viewcorresponding to that of the prior art plate shown in FIG. 1. In thecentral zone of the plate, a sealed-surface area 8 has been formed. Inthis area, two concentric O-ring seals 9, 10 of different diameters havebeen fitted, and in addition an opening 11 with sealed surfaces has beenformed. FIG. 5 shows a section analogous to FIG. 2. Through openings 11,an essentially pressureless unitary space 12 is formed, surrounding thepermeate channel and limited by the seal surfaces and sealed surfaceareas of the plates. The space 12 communicates with the outside.Permeate that has penetrated the membrane into the porous carrier bodycannot penetrate the sealed surface into space 12, but can only enterthe permeate channel, as the walls of the central bores are not sealed.If a leak occurs in seal 9, the liquid leaking through flows into space12 and out, indicating the presence of a leak. As the pressure in thepermeate channel is also greater than in space 12, also leaks occurringin the inner seals 10 are revealed, but the feed mixture is fullyprevented from entering the permeate channel. Space 12 is connected tothe outside e.g. via a tube through the filter housing.

The plates shown are circular, but other shapes may be used. Forexample, segments 13, 14 may be cut off as shown in FIGS. 1 and 5.

FIG. 6 shows a partial section through the end of another type of filterunit, along a plane parallel to the filtration circuit flow. This unitcomprises a tubular filter element 15, a housing head 16 and a seal 17between these. The end wall of the filter element is sealed by theimpermeable layer 24. The feed mixture enters at connection 18 and flowsunder pressure through the inner channels 20 of the element 15, coatedwith membrane 19. The permeate flows through the membrane and the porousbody 25 of element 15 into the shell space formed by housing wall 21.According to the invention, seal 17 comprises an internal leak detectionchannel system 22, formed by grooves in the upper and lower surfaces ofseal 17, said grooves being interconnected by evenly spaced openings.The channel system 22 communicates with the outside by channel 23,formed in housing head 16. As the feed mixture penetrates the inner partof the filter from the pressure side (18,20) of seal 17, it flows intochannel 22 and out via channel 23, indicating the presence of a leak,regardless of whether the leak flow occurs along the upper or the lowerface of the seal. The permeate side being normally under a higherpressure than the surroundings, permeate-side seal failures are likewiserevealed when the leaking fluid seeps out of channel 23.

In the embodiment of FIG. 7, seal 29 is provided between the end wall oftubular filter element 26 and the housing wall 27. The end surface 28 ofthe filter element and the wall surface section 30 at the site of filterelement are impermeably sealed. On the inner face of seal 29 areprovided groove 31 and at least one exit channel 32 connected to theoutside, providing a leak indicating channel system in the same manneras illustrated in FIG. 6. Possible leaks lead to flows from the jointbetween the housing head and the housing wall.

Automatic detection of leak flow may be provided in addition to orinstead of visual observation. For example, moisture sensors may beprovided in the intermediate spaces 12, 22, 31 or their correspondingexit channels, as shown schematically as 4 in FIG. 3.

The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A crossflow ceramic membrane filtering unitcomprising: a housing; one or more filtering elements coated with afiltering membrane on a filtering portion and having an impermeablesurface in a sealing region; joints between the filtering elementimpermeable surfaces or between the filtering elements impermeablesurfaces and the housing being sealed by means of a gasket system, thegasket system defining an inner seal and an outer seal with an internalintermediate space defined between the inner and outer seals; and ameans connected with the internal intermediate space to indicate a leakthrough one of the seals of the gasket system.
 2. The ceramic membranefiltering unit according to claim 1, wherein the intermediate space isconnected to the outside of the filtering unit.
 3. The ceramic membranefiltering unit according to claim 1 wherein the means includes: a sensorfor indicating fluid leaking into the intermediate spaces.
 4. Acrossflow ceramic membrane filtering unit comprising: a housing; one ormore plate-shaped filtering elements having permeate dischargeapertures, said elements being jointed together and to the housing, saidpermeate discharge apertures being formed through impermeable centralzones of the elements to define a conduit for permeate removal, theouter surface of the elements being coated with a filtering membrane anybeing essentially free to receive material to be filtered; theplate-shaped elements being fitted to each other and to the housing bymeans of at least two gaskets, the impermeable central zones of theelements forming intermediate spaces delimited by the gaskets and theimpermeable central zones, and said intermediate spaces beinginterconnected by leakage fluid apertures in the elements, said leakagefluid apertures having impermeable surfaces.
 5. A crossflow ceramicmembrane filtering unit comprising: a housing; at least one tubularfiltering element having at least one channel internally coated with afiltering membrane and at least one impermeable surface surrounding thechannel; a joint between the filtering element and the housing beingsealed by means of a gasket system, the gasket system including a pairof annular sealing surfaces which define an internal intermediate space,the pair of sealing surfaces defining a redundant seal between thehousing and impermeable surface of the filtering element; at least oneexit channel connected with the internal intermediate space to dischargeany fluid that bypasses one of the sealing surfaces.
 6. A method forsealing a ceramic crossflow membrane filtering unit which includes oneor more filtering elements within a housing, said elements being partlycoated with a filtering membrane and having an impermeable sealingsurface portion, the method comprising: sealing joints between at leastone of the impermeable sealing surface portions of the filteringelements and between the impermeable sealing surface portions of thefiltering elements and the housing by a gasket system which includes apair of seals with an intermediate space defined therebetween;monitoring or a liquid in the intermediate space to identify leakage inthe gasket system.
 7. The method according to claim 6 further including:filtering water through the filtering elements.
 8. The method accordingto claim 7 further including ultrafiltering the water.
 9. A crossflowmembrane filtering unit comprising: at least one filter element having afeed surface over which a feed liquid flows and a discharge surface fromwhich a permeate flow is discharged; a seal assembly disposed betweenthe filter element and one of: (1) another filter element and, (2) ahousing, the seal assembly including: a first seal portion in fluidcommunication with the feed liquid flow, a second seal portion in fluidcommunication with the permeate flow, and an intermediate space betweenthe first and second seal portions, the intermediate space being incommunication with a leakage discharge channel such that liquiddischarges through the discharge channel is indicative of leakagethrough at least one of the first and second seal portions.
 10. Thecrossflow membrane filtering unit according to claim 9 wherein the atleast one filter element includes: a plurality of filter plates, eachplate having an aperture defined adjacent the discharge surface, animpermeable surface surrounding the aperture, the feed surfacesurrounding the impermeable surface, the plates being stacked with pairsof the first and second seal portions disposed in a sealing relationshipbetween the impermeable surfaces of adjacent plates with one of theintermediate spaces being defined between each of the pairs first andsecond seal portions, the apertures defining a passage betweenintermediate spaces.
 11. The crossflow membrane filtering unit accordingto claim 10 wherein the first and second seal portions each include anannular gasket, the gaskets being mounted concentrically between theimpermeable surfaces of two adjacent filter elements such that anannular region between the gaskets defines the intermediate space. 12.The crossflow membrane filtering unit according to claim 9 wherein thefilter element includes: an interior bore extending between endsurfaces, the interior bore being surrounded by the feed surface; theend surfaces being impermeable, the seal assembly being disposed betweenat least one end surface and a housing in which the filtering element isreceived.
 13. The crossflow membrane filtering unit according to claim 9wherein the filter element includes: an interior bore defined by thefeed surface; a peripheral surface having an impermeable ring at leastat one end and the permeate surface defined thereadjacent; the sealassembly being disposed between the impermeable ring and a housing inwhich the filter element is received.
 14. The crossflow membranefiltering unit according to claim 9 further including: a sensor disposedin communication with the intermediate space for sensing a presence ofliquid in the intermediate space.
 15. A method of ceramic membranefiltering comprising: flowing a feed liquid over a feed surface of afilter element and discharging a permeate liquid from a dischargesurface of the filter element, the feed liquid and the permeate liquidbeing separated from each other by a seal assembly disposed between thefilter element and at least one of another filter element and a housingin which the filter element is received, the seal assembly having afirst seal for sealing the feed liquid from an intermediate space and asecond seal for sealing the permeate liquid from the intermediate space,the intermediate space being defined between the first and second seals;monitoring the intermediate space for the presence of liquid.
 16. Themethod according to claim 15 further including: in response to detectingliquid in the intermediate space, determining whether the liquid is thefeed liquid or the permeate liquid.
 17. The method according to claim 15wherein the feed liquid is heat sterilized water.